


there's no water inside this swimming pool

by kathillards



Category: Kamen Rider Build
Genre: Amnesia, M/M, Sparring Session, dealing with the memory switch
Language: English
Status: Completed
Published: 2018-06-21
Updated: 2018-06-21
Packaged: 2019-05-26 10:04:44
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,018
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14998505
Author URL: https://archiveofourown.org/users/kathillards/pseuds/kathillards
Summary: Ryuga—if he was being honest—wanted to see. To see if there was anything left of Sento, of his memories, inside this person he had never known. —- Katsuragi, Ryuga, and Sento in a circle that goes around and around.





	there's no water inside this swimming pool

**Author's Note:**

> takes place during episode 38, "mad world." honestly thought katsuragi would've lasted longer but since nobody wrote this while he was properly around, i obviously had to.

The lab feels distressingly empty when Ryuga chances stepping inside it that night.

Sento—no, Katsuragi—turns to look at him. There are papers strewn all over the floor and complicated equations all over the white boards. The computer lights drown the room in sickening silver-blue. In the middle of it all, Katsuragi stands, contemplative and confused, as if he’s come across a formula he can’t unlock.

Ryuga shakes off the déjà vu. This isn’t Sento; while the differences between the two of them are subtle, they’re enough to matter. Katsuragi holds himself differently, looks at his equations with a different eye, doesn’t look at him with—he doesn’t look at him the way Sento did, that’s the important part.

“Will you fight?” he asks Katsuragi, his voice echoing loudly in the room. Only the hum and buzz of the computers surrounds him. “Against Evolt?”

When he says it out loud, it seems like a stupid thing to ask. Katsuragi hates Evolt just as much as the rest of them do. But Gentoku had told him to go make sure—hadn’t wanted to do it himself, after whatever happened between him and Katsuragi earlier.

And Ryuga—if he was being honest—wanted to see. To see if there was anything left of Sento, of his memories, inside this person he had never known.

The look Katsuragi is sending him is cutting, thoughtful. “Did you think I wouldn’t?”

His voice is almost scornful. Ryuga bristles at it; he’d gotten used to Sento being arrogant in his own way, but _this isn’t Sento_.

“Well, you don’t know how to fight in Build,” Ryuga points out, walking around him to the other side of the lab, if only so he doesn’t have to look at this stranger in Sento’s body.

Katsuragi scoffs behind him. “I _built_ Build. And it should be muscle memory.”

Arrogant, so arrogant. Ryuga stops in front of the table at the back and curls his hands around the edge so he doesn’t turn around and punch Katsuragi. _You’re not Sento_ , he wants to scream. _You don’t know the things he knows_.

“It’s not that simple,” he says through gritted teeth. “You’re not used to that kind of power. Sento created RabbitRabbit and TankTank on his own. You—you might be a liability.”

When he turns around, Katsuragi’s eyes are narrowed, the expression hauntingly familiar and yet light years away from Sento. Ryuga expects a fight, almost braces himself for it. But Katsuragi doesn’t move.

He says instead, “When is the next battle?” Thoughtful, instead of angry.

Ryuga shrugs. “Tomorrow, probably. He’s going to go after Seito, and Nanba. Utsumi asked for our help.”

“And—what, you agreed to help them out of the goodness of your hearts?” Katsuragi’s head tilts, reevaluating him. Judging him. “All of you?”

“No.” Ryuga can’t help the scowl that crosses his face. “Kazumin doesn’t want to come. So it’s me and Gentoku… and you, if you’re up for it.”

Curiosity flashes across Katsuragi’s face. “Why doesn’t he want to come?”

“Because Seito killed his friends.” Ryuga knows he’s being curt, and Katsuragi knows it, too, because he crosses his arms and changes the subject.

“I’ll fight. Stopping Evolt is my—my responsibility. And I won’t be a liability.”

Ryuga looks him up and down carefully. He’s still got Sento’s body, but he carries himself differently. Sento was sure of himself, used to his height and muscles, always ready to jump into battle if he had to. Katsuragi isn’t used to that, isn’t comfortable in this body, and looks like he would shrink in front of a Smash before he would fight it.

He speaks before Ryuga can voice his concerns. “You’re worried I’ll hold you back.” It’s a statement, with an undercurrent of irritation that Ryuga would dare think he would be anything less than perfect.

“I’m worried you’ll get yourself killed,” Ryuga bites back, and in the next second knows what Katsuragi is going to say even before his eyebrows lower in a questioning frown:

“Why do you care?”

 _Why do you care?_ Ryuga stares at him helplessly, unable to even comprehend the question let alone explain the answer. The Devil’s Scientist wearing Sento’s face stares back at him, steady, calm, and curious. All Ryuga can do is try to stamp down the rising, burning ache deep inside him, the part of him that’s been screaming for Sento since the switch happened. Why does he _care_?

Because he’s Sento. He’s Sento’s body and Sento’s mind, even if he doesn’t have Sento’s memories or Sento’s heart. And Ryuga can’t fathom a world where he doesn’t care about Sento Kiryuu.

He says, “I wouldn’t want you to get Sento killed,” and adds just the right amount of scoff to his voice that Katsuragi might—without the aid of Sento’s memories to show him how to navigate the spikes and prickles of Ryuga Banjou—think he’s being serious.

Instead of rising to the bait, Katsuragi’s eyes, still narrowed, grow thoughtful. “You miss him.” Another statement, but this one more curious, more—wondering, maybe. Wondering how this had happened, this strange and chaotic relationship between himself and the man he was going to kill.

“Whatever.” Ryuga doesn’t want to deal with this conversation anymore, doesn’t want to deal with Katsuragi staring him down from across the lab and analyzing him and dissecting his like he’s a scientific equation. When Sento had looked at him like that, it had never felt so cold and callous; Sento had always looked at him with a warmth that Katsuragi will never be able to replicate, even on the same face.

“Wait,” says Katsuragi, just before Ryuga stalks past him to go up the stairs. “If you’re so worried… why don’t we go out and run a test experiment?”

Ryuga’s spent long enough at Sento’s side to be able to parse what this means with only a moment’s hesitation. “You want to fight me?”

Katsuragi looks defensive. “ _Training_.”

“It’ll hurt,” Ryuga warns him.

Katsuragi lifts his head, jaw set in stubbornness. He does, sometimes, look an awful lot like Sento, beyond them having the same face. “It’ll help.”

He sounds so sure of it, although Ryuga’s not clear on who exactly it will be helping—him, or Katsuragi. Either way, he shrugs and heads out, Katsuragi trailing behind him, until they reach the empty, abandoned park where he and Sento had had their fight right after they’d found out who Sento really was, with Misora watching over them.

Irony is something that keeps popping up in his life, Ryuga thinks ruefully, getting ready to transform. Even though Sento had to explain to him what it meant in the first place. And now he sees it everywhere, as he prepares to fight the man who was once his best friend, now just someone else who thought Ryuga needed to die.

He’s used to people thinking that, though. He stands there in the Cross-Z Dragon armor and waits for Katsuragi to get ready. Unlike Sento, his fingers are unsure on the Build Driver, and he cranks it slowly, as if waiting for something. Not at all prepared for an ambush, ready to leap into action at any moment the way Sento had been.

It aches and aches, but Ryuga pushes that ache away and focuses. Through the helmet of RabbitRabbit, Katsuragi stares him down.

Misora isn’t there to watch this time, so Ryuga lowers his inhibitions and just rams himself at him, fist raised and ready. Katsuragi, to his credit, meets him with an arm and dodges his kick, but he’s already allowed himself to be put on the defensive, which is how the fight gets away from you. Ryuga knows that better than anyone; the fight lives and breathes inside him, nurtured by his instincts and his tendency to bleed out before giving up.

He knocks Katsuragi down once, then twice a few moments later, and only feels about five percent of the punches Katsuragi lands on him. His form is off, his limbs unsteady and weak with the change of body, and there’s no way he’d last two seconds against Evolt.

Ryuga tells him this when he has him on the ground for the third time. “Evolt is going to kill you if you fight like this in the battle.”

He means it seriously, but for some reason, Katsuragi makes a noise that’s halfway between a cough and a laugh and says, confidently, “It’ll be easier in a real fight.”

Ryuga stares at him, disbelieving. Through the visor, it’s hard to see if Katsuragi is being smug or naïve. “No, it fucking won’t.”

“Studies show that in a real situation of danger,” Katsuragi begins, but Ryuga cuts him off.

“Shut up.” His voice is shaking, though he doesn’t know why. “Shut up, you can’t be so—so— _stupid_. This isn’t your body. This isn’t a _game_.”

Katsuragi has gone still underneath him. Ryuga feels his own armor melt off, his body trembling too much to hold the transformation, and then RabbitRabbit disappears in a flurry of red sparks, too.

“I assure you,” says Katsuragi, his voice very cold in a way Sento’s never has been, “I am not treating this as a _game_. But there is a logic to the system of Build, just like there’s a logic to Evolt’s madness, and a logic to everything—”

“There isn’t!” Ryuga snaps. “Not everything is _science_. There’s no logic to explain an alien monster from another planet trying to _destroy_ us.” He doesn’t add: _being a part of me, then not; taking things from my mind and spitting them back out; hiding inside the crevices of my body until I can’t trust myself around—_

_Around you._

Katsuragi’s gaze is scientist-sharp, skimming across Ryuga’s face, taking in the pain and the hurt and the anger that he can’t stop from being written all over. He should get up, he should leave Katsuragi to deal with his own damn system, he should stop being so close that he can hear Katsuragi’s breathing—Sento’s heart beating—but he doesn’t.

For some reason, Katsuragi doesn’t try to push him off either.

“You miss him,” he says, but this time it seems like even more has dawned upon him. “You and Sento—you—”

“Shut up,” Ryuga mutters again, and purposefully pushes himself off Katsuragi, scrambling to his feet with an unhidden wince. At least one of those punches had landed on his ribs. It’ll heal fast enough, but it still stings. He distracts himself with thinking about whether or not they have enough ice packs left to the point where he doesn’t notice Katsuragi has gotten up off the ground to follow him.

“Banjou,” he says, voice catching and holding him in place. “We can’t go into battle together if we’re fighting like teenagers. I’m only asking for the truth.”

“And I’m _telling you_ ,” Ryuga snarls at him, whirling back around with a vengeance not entirely deserved. “There’s no _logical_ truth here. It’s just—it just _is_. A lot of shit happened that you don’t remember! You can’t fight as well as us, you can’t fight like Sento did, you’re going to get yourself hurt on the battlefield or get someone killed or get _yourself_ killed and I—I can’t let that happen.”

Katsuragi has gone quiet, studying him again. Ryuga shifts his weight from foot to foot and tries to resist the urge to just turn around and walk away from that dissecting gaze as fast as he can. With Sento, it had been so different—more laughter, more wryness, more amused fondness. With Katsuragi, all it is, is trying to puzzle out all the little pieces that make up Ryuga Banjou and salvage them into someone worth knowing.

Sento hadn’t done that. Sento had known, instinctively, from the first moment, that Ryuga was someone worth knowing, and that was the real difference. That had made all the difference, from the very, very beginning.

Ryuga feels heavy from the weight of the knowledge that despite Sento’s eyes, and the quirk to Sento’s mouth, and Sento’s hair falling across his forehead, it’s not Sento looking at him. It’s not Sento seeing him, judging him, finding him someone worthy. It’s not Sento at all.

He feels the punch coming before he sees it. To Katsuragi’s credit, he’s actually managed a decent attack.

Ryuga knocks his arm away and retaliates with a spin and a kick, his body already jumping into action before even understanding what had happened. Katsuragi smiles grimly and goes for him again, their limbs crashing and interlocking and sliding muscles against muscles as they brawl.

“You think this’ll help?” Ryuga growls, pumped up on adrenaline as he throws Katsuragi off-balance with a well-aimed elbow. “Evolt’s not me.”

“Maybe not, but I think it’ll help you.” Katsuragi rams his hand into Ryuga’s back and the impact is strong enough that Ryuga groans. “You clearly have a lot of pent-up aggression towards me.”

“You tried to kill me and then I was framed for your _murder_ ,” Ryuga sputters in protest, but the actual heat of anger is missing from his words, absorbed by the intensity of the fight as he throws punch after punch, landing about half of them as Katsuragi dodges the rest. “Of course I have pent-up aggression towards you!”

“Right,” Katsuragi agrees, as calmly as if this was a normal sparring session between friends. “But it’s more than that, right? You’re mad at me for not being Sento.”

“I’m not mad,” Ryuga tries to say, but the distraction is sufficient enough that Katsuragi trips him over onto the grass and pins his arms down.

Ryuga takes a deep breath. “I’m not mad,” he says again, slower. Katsuragi is staring at him with Sento’s wide, earnest gaze, and it hurts more than the battle had. “It’s just that you don’t get it.”

“Get what?”

“Get _us_ ,” breathes Ryuga. Katsuragi is close enough, his brow furrowing, that he can see the ridge of his nose and the way his eyes—Sento’s eyes—crinkle at the edges when he’s lost in thought and trying to solve a particularly difficult problem. “You don’t—you won’t—”

“Show me,” Katsuragi demands, and for a second he looks and feels and speaks so much like Sento, so, so much like the man Ryuga had known, with his steady gaze and the press of _his_ body against Ryuga’s that—

Ryuga kisses him without thinking. Katsuragi’s entire body goes slack, releasing his hold on Ryuga’s arms in favor of soaking up the kiss. It’s not the best—hasty and wet and pressed haphazardly against his—Sento’s mouth, half out of anger and half out of a keen, desperate desire—but it lingers nonetheless when they part.

“…Oh,” says Katsuragi slowly.

Ryuga wants to shove him off but his arms won’t move. All he can do is lie there, heart pounding, looking up into Sento’s face—looking and looking and looking, waiting and praying for something, anything, a flash of recognition—

Maybe a part of him had hoped, stupidly and ridiculously, that kissing him would bring him back.

Katsuragi’s eyes glimmer in dawning awareness, and that hope rises again, burning white-hot in his chest.

But then Katsuragi says, “I get it now,” and the spell breaks with a vicious snap.

“Do you?” Ryuga sneers, and this time manages to push Katsuragi off him. He stops to think how pathetically easy that was; Sento is far more difficult to shove around, because he knows how to use his body weight properly, while Katsuragi is used to being lanky and nerdy and rolls off with only a surprised “Mmph.”

“Look,” says Ryuga carefully once he’s on his feet, brushing the dirt off his clothes. “We could use the help. RabbitRabbit and TankTank are two of the most powerful suits we got. So I won’t stop you from fighting. But—”

Here, he hesitates, looking back at Katsuragi who has pulled himself up to his knees, still on the grass, and is looking up at Ryuga with a look that says he gets it, but he doesn’t _get it_. Might never really _get_ Ryuga Banjou, not the way Sento does. And a look that says, maybe he regrets that, just a little.

Ryuga continues, “Just don’t pretend you’re Sento. You can’t do what he can. You don’t know us—me—Misora—any of us the way Sento does.”

“But especially you?” Katsuragi asks with a dry tilt to his lips, not quite a smile and not quite a frown. “I must say, I never considered that…” He stops for a moment to get to his feet and wipes his hands off. “I never considered that I—he—might actually fall in love with Evolt.”

Ryuga recoils. “I’m not Evolt,” is the only thing he can manage to say. His heart rattles in his ribcage, a lost and hopeless thing fluttering to snatch any one chance that—that—

“I suppose not,” Katsuragi agrees with a deep sigh as if Ryuga has gone horribly astray. In the glowing sunset light, he looks somehow both nothing like Sento and even more like him than he usually does, a paradox in two halves. “Regardless of what you are, I can see why he did what he did for you.”

His stomach is knotting itself up into tangles as Katsuragi steps closer, and closer. For a second, he’s afraid he’s going to kiss him again.

But all Katsuragi does is slide his gaze over Ryuga’s shoulder, to the hills and the horizon beyond him, and says with a quiet sort of shame, “I’ll get better before the fight, alright?” It’s half prickly and defensive, but half genuine. Ryuga can feel the astronomical effort it must have taken Katsuragi to put his ego aside to admit that.

He nods, once. “Alright,” he says. “You can at least practice with Kazumin.”

And before his voice can break or his body can betray him again, Ryuga turns and walks away, leaving Katsuragi to stare after him with Sento’s eyes, and Sento’s heart beating in his chest.


End file.
